The present invention relates to sound reproducing devices for producing stereophonic sound in narrow spaces, such as the passenger compartment of an automobile. More particularly, the invention relates to an improved sound reproducing device of this general type in which the stereo effect is enhanced.
When the listener listens to sound from such a sound reproducing device in the passenger compartment of an automobile, he feels as if the sounds are coming from a narrow room. This is the so-called "closed-room effect". Furthermore, in the passenger compartment of an automobile, the listener is near the the loudspeakers so that he hears sounds coming directly from the loudspeakers. That is, he hears the sounds as if he were near the sound source. This increases the closed-room effect even more.
Therefore, heretofore, in order to eliminate the above-described problems, signals having frequency components in middle and high sound frequency ranges, which give a high directional effect, were applied mutually to the opposite channels to decrease the directional effect and to thereby increase the acoustic spreading effect. In each channel, a signal obtained by delaying frequency components in a high range are applied to provide a reverberation effect, thereby to reduce the closed-room effect.
A conventional sound reproducing device which utilizes the above-described reverberation and spreading effects will be described with reference to FIG. 1.
In FIG. 1, reference character 1L designates an input terminal to which is applied a signal L (left) in a two-channel stereo system, 1R an input terminal to which is applied a signal R (right), 2 an addition circuit in which a signal-R which is obtained by inverting the phase of the signal R is added to the signal L, 4 a filter circuit, 3 a voltage amplifying or attenuating circuit for changing the amplitude of the signal L-R which has passed through the addition circuit 2, 5 an addition circuit in which the output of the filter circuit 4 (i.e. a signal L.sub.1 -R.sub.1) is added to the signal L from the input terminal 1L to output a signal L+L.sub.1 -R.sub.1, 6 an addition circuit in which a signal obtained by inverting the phase of the output signal L.sub.1 -R.sub.1 from the filter circuit 4 is added to the signal R from the input terminal 1R to output a signal R+R.sub.1 -L.sub.1, 7 a high-pass filter circuit, 8 a phase shift or delay circuit, 9 an addition circuit in which the output signal L+L.sub.1 -R.sub.1 from the addition circuit 5 is added to the output signal L.sub.2 -R.sub.2 from the phase shift or delay circuit 8 to provide a signal L+L.sub.1 +L.sub.2 -R.sub.1 -R.sub.2, and 10 an addition circuit in which a signal obtained by inverting the phase of the output signal L.sub.2 -R.sub.2 from the circuit 8 is added to the output signal R+R.sub.1 -L.sub.1 from the addition circuit 6 to output a signal R+R.sub.1 +R.sub.2 -L.sub.1 -L.sub.2.
Further in FIG. 1, reference character 11L designates a power amplifier circuit for the signal L+L.sub.1 +L.sub.2 -R.sub.1 -R.sub.2, 11R a power amplifier circuit for the signal R+R.sub.1 +R.sub.2 -L.sub.1 -L.sub.2, and 12L and 12R left and right loudspeakers, respectively. In FIG. 1, the signs (+) and (-) beside the addition circuits 2, 5, 6, 9 and 10 are intended to mean direct addition of a signal without phase inversion, and addition of a signal after its phase has been inverted, respectively.
As is apparent from FIG. 1, the two channel signals in the circuit are symmetrical. That is, when either of the two signals passes to the channel opposite to its own, its phase is inverted, but when it returns to its own channel, the phase is restored.
FIG. 2A shows a circuit P for simultaneously providing the two signals which are applied to addition circuits 5, 6, 9 and 10 with the phase of one of the signals maintained unchanged and the phase of the other inverted. In FIG. 2A, reference characters 13 and 14 designate resistors which are equal or substantially equal in resistance to each other, 15 a biasing resistor having a high resistance, Tr an NPN transistor, B2 the base terminal of the transistor Tr, C2 the collector terminal of the transistor Tr, E2 the emitter terminal of the transistor Tr, 16 an input terminal of the circuit P, 17 an inverted output terminal at which a signal obtained by inverting the phase of a signal applied to the input terminal 16 is provided, and 18 a non-inverted output terminal at which a signal applied to the input terminal is provided without being subjected to phase inversion.
FIG. 2B is an equivalent circuit Q of the circuit P shown in FIG. 2A. It may be noted that the biasing resistor 15 is not shown in FIG. 2B. In FIGS. 2A and 2B, like parts are designated by like reference numerals or characters. In FIG. 2B, reference character i designates a base current and Bi a current source. The current source Bi produces an output in a magnitude of B times the base current i to the emitter terminal E2.
Circuits P as shown in FIG. 2A are inserted between the input terminal 1R and the addition circuits 2 and 6, between the filter circuit 4 and the circuits which are the addition circuits 5 and 6 and the high-pass filter circuit 7, and between the phase shift or delay circuit 8 and the addition circuits 9 and 10.
The operation of the above-described circuitry for providing the reverberation and spreading effects will be described.
The phase of the signal R applied to the input terminal 1R is inverted by the circuit P to obtain the signal -R. The signal -R is added to the signal L from the input terminal 1L in the addition circuit 2 as a result of which the difference signal component L-R, representing the difference between the right and left signals, is provided. The signal L-R, after being amplified to a suitable value by the voltage amplifying or attenuating circuit 3, is applied to the filter circuit 4.
The difference signal component L-R includes a relatively large amount of reverberation component. This reverberation component is extracted by the addition circuit 2. From the reverberation component, the difference signal L.sub.1 -R.sub.1 having a frequency component in a range of about 100 Hz to 1.2 KHz, to which the ear is especially sensitive is filtered by the filter circuit 4 where a frequency component which produces a stronger reverberation effect is emphasized by a resonance circuit in the filter circuit 4. The signal L.sub.1 -R.sub.1 is added to the signal L in the addition circuit 5. Furthermore, the signal L.sub.1 -R.sub.1 is added to the signal R in the addition circuit 6, after its phase has been inverted by the circuit P. As a result, a reverberation effect and, expecially a spreading effect are provided by the sound from the loudspeakers 12L and 12R.
Next, the signal L.sub.1 -R.sub.1 having a relatively large amount of reverberation component, after passing through the filter circuit 4, is applied to the high-pass filter circuit 7, which is adapted to damp a low frequency component, where a signal component in a relatively high frequency range and having strong directional effect is extracted. The signal thus extracted is applied to the phase shift or delay circuit 8 where it is subjected to phase inversion or time delay to provide a signal L.sub.2 -R.sub.2. The signal L.sub.2 -R.sub.2 is added to the signal L+L.sub.1 -R.sub.1 in the addition circuit 9. Furthermore, the signal L.sub.1 -R.sub.1 is added to the signal R+R.sub.1 -L.sub.1 in the addition circuit 10 after its phase has been inverted. Because of the phase inversion and the time delay, a spreading effect and, especially, a reverberation effect are produced by the sound from the loudspeakers 12L and 12R.
As is apparent from the above description, the conventional sound reproducing device employs the circuit P as shown in FIG. 2 for obtaining the inverted outputs. The signal L.sub.1 -R.sub.1 from the filter circuit 4 is applied to the input terminal 16 of the circuit X. The signal L.sub.1 -R.sub.1, which is provided at the non-inverted output terminal 18 with its phase maintained unchanged, is added to the signal L applied to the addition circuit 5 from which the resultant signal L+L.sub.1 -R.sub.1 is applied to the addition circuit 9. On the other hand, the signal R.sub.1 -L.sub.1 having its phase inverted is provided at the inverted output terminal 17. The signal R.sub.1 -L.sub.1 thus provided is added to the signal R from the input terminal 1R in the addition circuit 6 from which the resultant signal R+R.sub.1 -L.sub.1 is supplied to the addition circuit 10.
With the circuits P connected so that the signals are processed as described above, when the signal L applied to the addition circuit 5 from the input terminal is introduced to the addition circuit 9, a small part of the signal L will leak into the addition circuit 6. Similarly, when the signal R applied to the addition circuit 6 from the input terminal 1R is introduced to the addition circuit 10 a small part of the signal R tends to leak into the addition circuit 5 from the addition circuit 6. However, leakage of the signal R is never caused because of the presence of the circuit P. That is, the left signal L leaks into the opposite channel but the right signal R does not because of the provision of the circuit P. This will be described with reference to the equivalent circuit Q of the circuit P.
When the small part of the signal L, i.e. the small leakage signal, passes to the equivalent circuit Q from the non-inverted output terminal 18, it affects the base current i and therefore the current source Bi is also affected thereby. Thus, the leakage current flows from the emitter terminal E2 to the collector terminal C2 and to the inverted output terminal 17. In other words, a small leakage signal L' flows from the addition circuit 5 to the addition circuit 6.
When part of the signal R flows to the circuit Q from the inverted output terminal 17, it can reach the collector terminal C2 but it cannot affect the base current i, and therefore the current source Bi is not affected thereby. Thus, leakage current will not appear at the emitter terminal E2. In other words, part of the signal R, i.e. the leakage current, will not flow from the circuit 6 to the circuit 5.
The above-described phenomenon occurs between the addition circuits 9 and 10 also. A part of the signal L+L.sub.1 -R.sub.1 applied to the addition circuit 9 from the addition circuit 5, i.e., a small leakage signal L"+L.sub.1 "-R.sub.1 ", leaks into the addition circuit 10 from the additional circuit 9.
Because of the above-described phenomenon, the sound from the right and left loudspeakers is not symmetrical. Even if the difference signal output is made zero, the leakage of signals cannot be prevented without providing a circuit which completely separates the right and left signal paths. Therefore, the leakage of signals make the asymmetry of the sound from the right and left loudspeakers and the leakage of signals make bad the degree of separation between the right and left channels.
Moreover, if the difference output signal is not zeroed, the conventional sound reproducing device suffers from a drawback that the asymmetry of the right and left frequency characteristics is increased, especially, in the middle and high frequency ranges.